BOSTON - A cancerous brain tumor caused the seizure Sen. Edward M. Kennedy suffered over the weekend, doctors said Tuesday in a grim diagnosis for one of American politics' most enduring figures.

Doctors for the Massachusetts Democrat say tests conducted after Kennedy suffered a seizure this weekend show a tumor in his left parietal lobe. Preliminary results from a biopsy of the brain identified the cause of the seizure as a malignant glioma.

His treatment will be decided after more tests but the usual course includes combinations of radiation and chemotherapy.

"I'm really sad," former Sen. Bob Kerrey, D-Neb., said when told in a Senate hallway about Kennedy's condition. "He's the one politician who brings tears to my eyes when he speaks."